Metal-halide discharge lamps usually are of intermediate or relatively high-wattage, such as 175 to 1500 watts. The luminous efficacy of such lamps decreases as the wattage of the lamp decreases. It had generally been believed that at wattages of 100 watts or less, metal-halide lamps would be unsatisfactory insofar as efficacy is concerned.
It is common practice in intermediate and relatively high-wattage lamps to provide an inert fill gas within the outer envelope in order to prevent oxidation of metal parts of the arc tube mount. Another advantage of the inert gas fill within the outer envelope is a high breakdown voltage which prevents arcing between metal parts of the arc tube mount. There is, however, an undesired heat loss due to convection currents of the inert gas within the outer envelope which reduces the lamp efficacy significantly, particularly with lower wattage lamps.
One known attempt to reduce the undesired heat loss due to convection currents within the outer envelope is disclosed in U.S. Pat. Nos. 4,499,396, and 4,580,989, both to Fohl et al. and assigned to the assignee hereof. Therein, a domed quartz sleeve is disposed within the gas-filled outer envelope of a metal-halide discharge lamp such that convection currents are suppressed and convective heat loss is substantially reduced. In U.S. patent application, Ser. No. 621,648, filed June 18, 1984 and assigned to the assignee hereof, there is disclosed a metal-halide discharge lamp having a light-transmissive enclosure about the arc tube within an evacuated outer envelope. The disclosure of 621,648 teaches that various temperatures over the body of the operating arc tube increase nonuniformly when an arc tube enclosure is employed in combination with an evacuated outer envelope. The hot spot temperature increases to a lesser extent than the cold spot temperature, so that the distribution of operating temperatures over the body of the arc tube is more nearly isothermal resulting in improved lamp performance. The disclosure of 621,648, however, provides no guidance on the choices of physical parameters for the enclosure vis-a-vis the arc tube in order to optimize the benefits of heat conservation and redistribution in an evacuated outer envelope.
The state of the art has advanced to the point where lower wattage metal-halide lamps are commercially feasible. Nevertheless, it would be a substantial contribution to the art if there were provided a lamp structure which optimized performance characteristics in metal-halide lamps of various wattages, particularly in lower wattage lamps.